Techniques for reducing wall thinning during a hydroforming operation

ABSTRACT

It has been found that a compressive force assists in obtaining desired cold flow characteristics of a tubular steel work piece during a hydroforming operation. It has also been found that the most desirable results are achieved when the compressive force applied to the ends of the tubular steel work piece is just slightly less than the force that drive the tubular steel work piece into its yield point. A work piece previously formed by the above mentioned hydroforming operation may now be bent and subjected to a secondary hydroforming operation to produce a bent hydroformed tubular steel finished work piece having complex geometry.

This application is an improvement over U.S. Pat. No. 6,912,884.

BACKGROUND OF THE INVENTION

U.S. Pat No. 6,912,884 describes a method of hydroforming tubular objects which overcomes some of the problems of the prior art. For instance, where tubing is expanded by mechanical means, the problem of localized tube wall thinning is a persistent problem. The above patent provides a method of hydroforming a tube having enlarged sections where the enclosing die set is manipulated to undergo controlled movement so as to reduce the wall thinning phenomena during a hydroforming operation. This is often accomplished by controlled die movement and applying compressive force to the work piece whilst a hydroforming operation is taking place.

SUMMARY OF THE INVENTION

It has been found that the material of the work product may be made to “cold flow” in a predetermined manner by altering. the pressure applied to the ends of the tube before hydroforming pressure is applied. If the pressure applied to the ends of a tube is slightly below the yield point of the steel comprising the work product which is undergoing a hydroforming operation, it will be found that when a hydroforming pressure is applied to the work product, the previous problem of “localized wall thinning” is reduced. All thinning occurs because of uneven material flow in the various sections of the work product undergoing the hydroforming operation. “Preloading” of the work at the ends of the work piece to produce the axial compressive force will certainly ameliorate the problem of wall thinning.

At the same time it is possible to introduce an intermediate cold forming pre-wrinkling hydroforming operation on a work product before the final hydroforming operation is attempted. In this manner controlled “wrinkles” are introduced into the surface of the article about to be hydroformed in the area where maximum thinning of the wall of the hydroformed work piece is usually expected.

In a last attempt to reduce the phenomena of “wall thinning”, it will be found possible to initiate a preliminary hydroforming technique on the work product before a process of final shaping of the work product is initiated. In this manner a work product which is to ultimately undergo a bending operation is subjected to at least one preliminary expansion operation of the work piece before any subsequent operation on the work piece occurs. This is followed by bending the expanded tube to a predetermined shape by any suitable prior art bending technique, and subsequently performing a final hydroforming operation on the previously shaped work piece while taking advantage of any previous hydroforming operation in which an intermediate work product is produced. It is possible to produce a work piece in which sections which are subject to failure because of “wall thinning” may be thickened in predetermined areas. This results in a final hydroformed product having substantially even wall thickness after the final hydroforming operation.

PERTINENT PRIOR ART

-   U.S. Pat. No. 6,912,884 -   U.S. Pat. No. 6,397,449

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 a represents a prior art apparatus in which the dies surround the work product are movable and the work product is preloaded with an axial force.

FIG. 1 b shows an apparatus of FIG. 1 a with the dies closed and the work product has been hydroformed to fill the work cavity.

FIG. 2 shows a sectional view of a composite die set for hydroforming a “wavy” surface on the work product.

FIG. 3 a illustrates a die set (open) before a tube is inserted.

FIG. 3 b illustrates a die set (closed).

FIG. 3 c shows a tubular work piece which has been formed in a hydroforming. operation using the apparatus of FIG. 3 a and 3 b.

FIG. 4 shows the work piece of FIG. 3 c which was previously hydroformed and has been bent and is now placed in a die for a subsequent hydroforming operation.

FIG. 5 a shows a tube which has been hydroformed so as to have a “wavy” shape.

FIG. 5 b shows the tube of FIG. 5 a now bent to a predetermined shape.

FIG. 5 c shows the tube of FIG. 5 a and 5 b after a subsequent hydroforming operation.

DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 shows a hydroforming die set 10 which is composed of two sections, 12 and 14. As in the prior art, the die 10 is intended to separate at 16 for work removal etc.

A tube 18 is inserted into die 10 to be hydroformed in recess 20. A small gap 16 is left between die sets 12 and 14. In the usual hydroforming operation tube 18 is enlarged in chamber 20 merely by applying hydraulic pressure to the inside of tube 18. In this instance a compressive pressure P is also applied to the ends of tube 18. This puts the tube 18 in a suitable confinement for expansion when a suitable hydraulic force is applied to the interior of tube 18. Gradually the gap shown at 16 is reduced to zero as the tube expansion progresses and the compressive force P is continuously applied to the ends of tube 18.

Because of the previously applied force P, the tube 18 may now be formed into a predetermined shape without substantial wall thinning as would be present in the absence of the pressure P.

FIG. 1 b shows a die set 10 having tube 18 expanded and gap 16 closed. The expanded tube may now be removed from the die set 12, 14.

The compressive force P which may be ultimately applied to the ends of tube 18 is a variable, but it has been found that in hydroforming work products of a shape shown in FIGS. 1 a and 1 b that the expansion of tube 18 may best be done when the compressive force P is just slightly below the yield point of the steel tubing 18. This produces a minimum of wall thinning of tube 18 when compared to the prior art techniques.

Referring now to FIG. 2 it will be seen that a composite die 110 comprises at least four subassemblies 112, 114, 122, 124 (not shown). Here the end pieces 112 and 114 are configured to fiction in a manner similar to die pieces 12 and 14 of FIGS. 1 a and 1 b. However the mid pieces 122 and 124 (not shown) must separate along the longitudinal axis to permit removal of the ultimately formed tube. A cavity having a wavy surface 120 is formed in center sections 122 and 124. For the successful production of an expanded work piece having a wavy surface it will be expedient to utilize a compressive force P on the ends of tube 118 which is just slightly less than the yield point force for steel tube 118. This produces expansion of tube 118 with a minimum of wall thinning in the critical areas.

FIGS. 3 a and 3 b show a die set 310 comprising die pieces 312, 314, 326 and 328. These dies together are used to produce the expanded tube 318 as shown in FIG. 3 c. It may be that pressure will have to be applied to the tube at the end dies 312 and 314 to prevent unwanted thinning of the tube 318 during the hydroforming operation.

The tube 318 as shown in FIG. 3 c is thus bent to the shape to fit in die set 410 so that a subsequent hydroforming operation may be performed on the tube 318 to produce the desired shape. Here. bulges 420, 422 and 424 are produced in tube 318 during the subsequent hydroforming operation.

FIGS. 5 a, 5 b and 5 c show the progression of a “wavy” tube 518 which has previously exposed to a hydroforming operation similar to that shown in FIGS. 1 a and 1 b to produce “waves” such as shown at 520 in tube 518. The tube 518 is subsequently bent at predetermined locations 522 and 524 to a form that will fit into die set 510. The tube 518 is then hydroformed to produce the desired cross section for tube 518. Here expanded sections 530, 532 and 534 are produced by the subsequent hydroforming operation.

It will be understood that die sets 410 and 510 are conventional die sets comprising 2 dies mated together (so that tubes 418 and 518 may be conveniently removed from the die sets 410 and 510).

This technique has permitted the formation of straight tube into hydroformed shapes having a variety of cross sections wherein the previous thinning of the various expanded sections is avoided. This was thought to be impossible to achieve.

Although variations of the hydroforming will become apparent to those skilled in the art, applicant will rely on the following claims to protect the inventions disclosed herein. 

1. A hydroforming operation comprising: selecting a suitable tubular steel work piece having two ends, subjecting said work piece to a preliminary hydroforming operation in which said work piece is placed into a presselected die set comprising at least two movable die components where an axial gap exists prior to said preliminary hydroforming operation; expanding said tubular work piece in said die set whilst simultaneously applying a compressive force which is slightly less than a yield point force for said tubular steel work piece is applied to the ends of said tubular steel work piece; whilst closing said gap by moving said movable die components together; removing said work piece from said presselected die set at the conclusion of said preliminary hydroforming operation, bending said work piece to form a predetermined shape; placing said bent work piece in a second die set performing a second hydroforming operation on said work piece whilst said work piece is in said second die set.
 2. A method of hydroforming a tube to a predetermined shape comprising; providing a first die set comprising members of a peculiar configuration having a particular shaped internal cavity shaped therein; and said first die members being spaced apart to have a space between said dies, inserting said tube into said first die set and, applying hydraulic pressure to the tube to cause said tube to conform to the shape of said cavity whilst simultaneously: applying a predetermined compressive force to the ends of said tube and moving the first die set members together to eliminate said spacing between said first die members.
 3. The method of claim 2 wherein said predetermined force is slightly less than the force necessary to cause said tube to yield.
 4. The method of claim 3 wherein said tube is removed from said first die set cavity, and is bent into a second configuration and inserted into a second die set having a second cavity therein; and applying a second hydraulic pressure to said tube to cause said tube to conform to said second cavity, and removing said tube from said second die set. 